Optical elements having buried layers

ABSTRACT

A buried conductive and/or reflective layer is provided in an optical element including at least one chemically vapor deposited material layer. Over a first layer of material is provided an intermediate region. The intermediate region in one embodiment includes at least one layer of a refractory-type material. In an alternate embodiment, the intermediate region is a composite intermediate region including a first passivating layer comprising a layer of a refractory-type of material such as one of the borides, carbides, nitrides, oxides and silicides, or a refractory-type of metal such as tungsten, molybdenum, tantalum, titanium and rhodium or a refractory-type of metal alloy. A conductive layer is then provided over at least a portion of the first passivating layer. Said conductive layer may comprise any of the highly conductive/reflective metals such as copper, gold, silver, palladium, platinum and aluminum, for example. Over the conductive layer is provided a second passivating layer similarly comprising one of the aforesaid mentioned refractory-type of materials. A second layer of material is then chemically vapor deposited over the intermediate region. The single layer of refractory material provides an intermediate region having acceptable optical characteristics subsequent to chemical vapor deposition. The first and second passivating layers of the composite intermediate region isolates the conductive layer from the materials preventing interdiffusion and chemical reaction therebetween. Further, the second passivating layer protects the conductive layer from the generally corrosive and reducing or oxidizing environment provided during chemical vapor deposition of the second layer of material.

The goverment has rights in this invention pursuant to Contract No.F29601-82-C-0103 under Subcontract 04-427856-UPT which was awarded bythe Department of the Air Force.

BACKGROUND OF THE INVENTION

This inveniion relates generally to optical elements, and moreparticularly, to infrared optical elements.

As is known in the art, optical elements and particularly infraredoptical elements such as windows, domes and lenses are often comprisedof material which is fabricated from a process known as chemical vapordeposition. The chemical vapor deposition (CVD) process generallyincludes the steps of directing reactant gases into a reactor vesseldisposed at an elevated temperature and chemically reacting said gasesto form the material. The material is deposited over a substrate toprovide the particular optical element. The CVD process is generally acontinuous process in which new reactant gases are introduced into thevessel and by-product gases and undeposited material vapors are vented.Generally, the combination of the reactants and by-product gases as wellas the elevated temperatures provide a highly corrosive and possiblechemically reducing or oxidizing environment.

It would be desirable in many applications to have optical elementshaving either layers or gratings buried within the optical element. Suchlayers or gratings may be used for heating to de-ice the opticalelement, provide electromagnetic shielding, or provide electromagneticabsorption. These layers may also be used to provide a surfacereflective to one or more wavelength ranges of incident electromagneticenergy. Optical elements such as bandpass filters and, dichroic beamsplitters which require a pair of surfaces reflectively responsive todifferent electromagnetic wavelengths, could be fabricated having one ormore buried reflective layers.

Nevertheless, as mentioned above, when the optical element is fabricatedby providing a chemical vapor deposited material over a reflectiveand/or conductive surface, the above-mentioned high temperature andchemically reducing environment generally will degrade the surfacemorphology of most of the highly reflective and/or conductive materials.Moreover, when layers of the highly reflective metals such as gold orsilver are provided, the high temperature and corrosive environment ofthe chemical vapor deposition process generally causes the layer toagglomerate. When first deposited, these metals tend to have amirror-like, smooth and hence reflective surface. However, duringchemical vapor deposition, small islands of the material are formedleaving behind holes previously occupied by the material. Moreover, forsome materials such as silver, total removal of the layer often occurs.This degradation in the surface morphology leads to reduced conductivityand/or reflectivity of the buried layers. Typically, high conductivityand/or reflectivity are the most important properties of these buriedlayers. Accordingly, buried layers comprising highly reflective and/orconductive meaterials are typically not found within optical elementsfabricated from chemically vapor deposited materials.

SUMMARY OF THE INVENTION

In accordance with the present invention, an optical element includes abase layer, an intermediate layer disposed over said base layercomprising a refractory material, and an overcoat layer disposed oversaid intermediate layer comprising a layer of a compatible chemicallyvapor deposited material. The refractory material may comprise arefractory type of metal such as tungsten, molybdenum, tantalum,titanium and rhodium or a refractory dielectric such as one of theborides, carbides, nitrides, oxides or silicides. The selection of thematerial of the refractory layer is determined by the desired opticalproperties of the element. For example, a refractory type of metal maybe provided over the base layer and patterned to provide a heating gridto de-ice the optical element during operation. Alternatively, one ofthe aforesaid refractory dielectrics may be chosen based upon opticalproperties which permit such a layer to be transparent or absorptive toelectromagnetic energy of a first wavelength band and reflective toelectromagnetic energy of a second, different wavelength band. With thisarrangement, by providing the refractory layer sandwiched between thebase layer and the chemically vapor deposited overcoat layer, an opticalelement is provided having an intermediate layer having a high degree ofresistance to surface morphology degradation during chemical vapordeposition of the overcoat layer. With such high resistance tomorphological change during chemical vapor deposition, the reflectiveand/or conductive properties of such layers are substantially unaffectedby chemical vapor deposition of the overcoat layer. Furthermore, forcertain materials such as the refractory type metals, the reflectance oflayers of these materials may actually increase after chemical vapordeposition due to annealling and grain growth of the layer duringchemical vapor deposition of the overcoat layer.

In accordance with an alternate embodiment of the present invention, anoptical element includes a first base layer, a composite intermediatelayer comprising a first passivating layer of a refractory materialdisposed over said first base layer, a conductive layer disposed oversaid first passivating layer, and a second passivating layer of arefractory material disposed over said conductive layer. The opticalelement further includes an overcoat layer comprising a chemically vapordeposited material disposed over the second passivating layer. With suchan arrangment, the pair of passivating layers protect the conductivelayer disposed therebetween from the reducing and corrosive environmentgenerally encountered during chemical vapor deposition. Further, thepassivating layers also protect the conductive layer from diffusion andreaction with the material of either the base layeroor overcoat layer.

In accordance with a further aspect of the present invention, the pairof passivating layers comprise a refractory type of metal or arefractory dielectric. The refractory type of metal may includetungsten, molybdenum, tantalum, titanium and rhodium, as well as alloysof said metals, whereas, the refractory dielectric may include one ofthe borides, carbides, nitrides, oxides or silicides. More particularly,the refractory dielectric may include beryllium oxide, aluminum oxide,silicon dioxide, thorium oxide, yttrium oxide and zirconium oxide, themost preferred refractory dielectric being beryllium oxide. Thepassivating layers each have a thickness selected to completely isolatethe conductive layer from the base layer and overcoat layer. Theconductive layer may include a metal, more particularly, a refractorytype of metal such as tungsten or one of the highly conductive metalssuch as copper, silver, gold, platinum and palladium or a metal such asaluminum. With such an arrangement, the refractory material comprisingthe passivation layers protects and isolates the conductive layer fromthe elevated temperature and chemically corrosive and potential reducingor oxidizing environment generally encountered during chemical vapordeposition of the overcoat layer. Furthermore, the passivating layersprevent reaction between and interdiffusion between the conductive layerand the material of the base and overcoat layers. Further still, withcertain materials such as platinum and tungsten, for example, thereflectivity of the material over the wavelength range of 2 to 16microns increases after chemical vapor deposition of the overcoat layer.It is believed that while the passivation layers prevent chemicalreaction between the conductive layer and reactants present duringchemical vapor deposition of the overcoat layer, the elevatedtemperatures present during growth of the overcoat layer cause theconductive layer to be annealed between the pair of passivating layerscausing grain growth of the material of the conductive layer, therebyincreasing the reflectivity of such layer above the reflectivityencountered prior to providing the overcoat layer.

In accordance with a further aspect of the present invention, a methodof forming an optical element having a buried layer includes the stepsof: providing a base layer, depositing an intermediate layer over asurface of said base layer, said intermediate layer comprising at leasta first layer of at least a first refractory type of material, andchemically vapor depositing an overcoat layer over the intermediatelayer. With such an arrangement, the refractory material is selected towithstand the high temperatures and chemically reducing environmentencountered during chemical vapor deposition to provide a buried layerwithin the optical element. The intermediate layer may comprise arefractory type of metal such as tungsten (W), molybdenum (Mo), tantalum(Ta), titanium (Ti), rhodium (Rh) or other refractory types of metals ora refractory type of dielectric such one of the borides, carbides,nitrides, oxides or silicides. The optical element can be fabricatedhaving internal heating, shielding, absorption or reflectioncharacteristics in accordance with the characteristics of the layer.Furthermore, if multiple layers are provided, the optical element mayhave selected band-type characteristics or combinations of the aforesaidcharacteristics

In accordance with a still further aspect of the present invention, amethod of forming an optical element having a buried passivatedconductive layer includes the steps of: providing a base layer,depositing a composite intermediate layer over said base layercomprising consecutively deposited layers of a refractory material, aconductive material, and a second layer of a refractory material, andchemically vapor depositing an overcoat layer over the second layer ofthe refractory material. With such an arrangement, the refractorymaterials are selected to withstand the high temperatures and chemicallyreducing environment encountered during chemical vapor deposition of theovercoat layer, thus protecting the conductivity and reflectivity of theconductive layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of this inventinn, as well as the inventionitself, may be more fully understood from the following detaileddescription of the drawings, in which:

FIG. 1 is an isometric view of an optical element, here a plateincluding a base layer, an intermediate layer and an overcoat layer;

FIG. 2 is a cross-sectional view taken along line 2--2 of FIG. 1 showingthe intermediate layer oomprising a buried dielectric layer inaccordance with one aspect of the present invention;

FIG. 3 is a cross-sectional view taken along line 3--3 of FIG. 1 showingthe intermediate layer comprising a buried conductive layer inaccordance with an additional aspect of the present invention;

FIG. 4 is a cross-sectional view taken along line 4--4 of FIG. 1 showingthe intermediate layer comprising a composite intermediate layerincluding a conductive/reflective layer disposed between a pair ofpassivation layers in accordance with a further aspect of the presentinvention;

FIG. 5 is a cross-sectional view taken along line 5--5 of FIG. 1 showinga composite intermediate layer including a conductive/reflective layerdisposed between a pair of passivation layers and a dichroic reflectivecoating disposed over a surface of the overcoat layer in accordance witha still further aspect of the present invention;

FIG. 6 is a cross-sectional view taken along line 6--6 of FIG. 1 showinga composite intermediate layer including a patterned layer disposedbetween a pair of passivating layers in accordance with a still furtheraspect of the present invention;

FIG. 7 is an exploded cross-sectional view taken along line 7--7 of FIG.6;

FIG. 7A is a plan view of FIG. 7;

FIGS. 8-9 are graphs of percent reflectance vs. wavelength for varioustypes of buried layers.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, here an optical element 10 is shown to includea pair of layers 12, 18, each comprising a material having selectedelectromagnetic and in particular optical properties. The opticalelement 10 further includes an intermediate region 13 disposed orsandwiched between layers 12 and 18. Intermediate region 13 may have oneof several different types of construction in accordance with thefunction of the optical element 10 as will be described in conjunctionwith FIGS. 2-6. Suffice it here to say, however, that the opticalelement 10 may be, for example, an optical window 11 having aselectively reflective dielectric layer 14 (FIG. 2), an optical window11' having a conductive/reflective layer 14' (FIG. 3), an optical window11" having a buried composite layer 15 (FIG. 4), a plate-type dichroicbeam splitter 31 (FIG. 5) having the buried reflective dielectric layer14 (FIG. 2) or the buried conductive reflective layer 14' (FIG. 3), oran rf absorbent, infrared transparent window 32 having a buriedcomposite layer 15' including a patterned rf absorbent layer 15b' (FIG.5). Further, the optical element may also be any of other types ofelements such as a lens, mirror, filter, polarizer, prism or the like.

In any event, here the optical element 10 includes the first substrateor base layer 12 preferably comprising a chemically vapor depositedmaterial. The intermediate layer 13 here comprises either a singledielectric/reflective layer 14 (FIG. 2A), a single conductive/reflectivelayer 14' (FIG. 2B), a composite layer 15 (FIGS. 3 and 4) comprising aconductive layer disposed between a pair of passivating layers 15a, 15b(FIG. 4) or a composite layer 15' (FIGS. 6, 7, 7A) having a patternedconductive layer 15b' disposed between a pair of passivating layers 15a,15c'. The second or overcoat layer 18 is here comprised of a chemicallyvapor deposited material and is deposited over the intermediate layer 13to provide the optical element 10. The material of substrate or baselayer 12 and substrate or overcoat layer 18 each comprises any suitableoptical material preferably comprised of a chemically vapor depositedmaterial. The chemically vapor deposited material is characterized byhaving an actual density substantially equal to 100% of the theoreticaldensity of the material, is substantially pore-free, substantiallystress-free and is generally optically transmissive to electromagneticenergy having wavelengths up to typically 30 microns. Furthermore,layers of the chemically vapor deposited material typically havethicknesses in excess of 25 microns, although layers having smallerthicknesses may be fabricated. Examples of suitable chemically vapordeposited materials having known optical characteristics include zincsulfide (ZnS), zinc selenide (ZnSe), gallium arsenide (GaAs), mercurycadmium telluride (HgClTe), germanium (Ge), silicon (Si), galliumphosphide (GaP), aluminum oxynitride (Al₃ ON₂) (ALON), yttrium oxide (Y₂O₃), magnesium oxide (MgO). Other suitable materials nevertheless mayalternatively be used in accordance with this invention. Fabrication ofparticular examples of optical elements 10 will now be described inconjunction with the cross-sectional views of FIGS. 2-6.

Referring first to FIG. 2, optical element 10 (FIG. 1), here a filterelement 11 is shown to comprise a base layer 12, here comprising one ofthe aforesaid mentioned optically transmissive materials which ispreferably a material formed by chemical vapor deposition. Thereflective layer 14, here comprises a refractory material which issubstantially chemically inert during chemical vapor deposition (CVD) ofovercoat layer 18, and has a relatively high melting point temperature,typically in excess of the temperature encountered during CVD.Furthermore, the refractory material should have optical and physicalproperties which are compatible with the material of base layer 12, andthe overcoat layer 18 (as will be described hereinafter). Further, therefractory material should have properties which are compatible with theapplication or function of the optical element. Also, after fabrication,the refractory material should be chemically inert and hence chemicallycompatible with the materials of layers 12 and 18. The reflective layer14 is here comprised of a refractory material, more particularly, eithera refractory-type of metal (as will be described in conjunction withFIG. 3) or a refractory dielectric as will now be described.

The refractory dielectric comprising the reflective layer 14 is heresputtered over the base layer 12. Alternatively, the layer 14 may bedeposited by other known techniques and optionally may be patterned. Thematerial is typically deposited to a thickness generally exceeding 1micron. The reflective layer 14 is here comprised of a refractorymaterial, more particularly, at least one material selected from thegroup consisting of the borides, carbides, nitrides, oxides orsilicides. More particularly, the refractory material may comprise amaterial having such refractory characteristics and further a materialwhich exhibits the so-called "residual ray" or "Reststrahlen effect."Thus, materials having predetermined known spectral properties may beused as the layer to provide optical elements having filtering and, inparticular, bandpass filtering characteristics. Examples of suitablematerials are listed in The Handbook of Optics, by W. G. Driscoll,Editor, McGraw-Hill Book Company, New York, NY, pp. 8-97, Table 4. Whenlayer 14 is selected to be a continuous layer of one of the aforesaidmentioned Reststrahlen effect materials, the optical window 11 may haveband pass/band reject filter characteristics. For example, berylliumoxide BeO is substantially transparent to electromagnetic energy havinga wavelength of less than about 7 microns, is reflective over a range ofabout 7 to 13 microns, and is absorptive for wavelengths greater thanabout 13 microns. Accordingly, by providing a single layer 14 comprisingberyllium oxide, a band pass/reject filter is provided which issubstantially reflective of relatively long wave infraredelectromagnetic (LWIR) energy and is substantially transparent to shortwavelength infrared and visible electromagnetic energy.

An example of the optical element 11 described in conjunction with FIG.2 was fabricated having the single dielectric reflective layer. Thedielectric layer was ion sputtered over the base layer 12. However,other suitable deposition techniques for the dielectric layer 14 mayalternatively have been used.

    ______________________________________                                        Material of Layers 12, 18                                                                           ZnSe                                                    Deposition Temperature of Layer 18                                                                  700° C.                                          Thickness of layers 12, 18                                                                          0.200 inches                                            ______________________________________                                    

                  TABLE 1                                                         ______________________________________                                        Example No.                                                                            Material of Layer 14                                                                          Thickness % R                                        ______________________________________                                        1        BeO             2.8 microns                                                                             FIG. 8                                                                        curve 50                                   ______________________________________                                    

The percent reflectance (%R) is shown in FIG. 8 curve 50. For the rangeof 9.5 to 13 microns, the percent reflectance is at least 80%.Measurements of percent reflectance were taken through the overcoatedlayer 18 of ZnSe as reflected from the reflective layer 14. APerkin-Elmer spectrophotometer, Model 580-B was used to take themeasurements using the front side mirror surface of an aluminum plate asa reference. The varying level of reflectance below about 7.0 microns,curve region 50a, is due to two effects. The d.c. type level at about30%R is due to the high index of refraction of ZnSe, and the sinusoidtype of variation is the result of interference patterns created by theZnSe and BeO interface. This level could be reduced, typically, to thelevel shown by phantom line 51 by a suitable conventional broadbandantireflection coating over the overcoat layer 18.

Referring now to FIG. 3, an optical element 10 (FIG. 1) here a window11' is shown to include the base layer 12 comprising one of theaforesaid materials and a conductive layer 14' comprising a refractorytype of metal such as tungsten (W), molybdenum (Mo), tantalum (Ta),rhodium (Rh), titanium (Ti) and other refractory types of metals.Refractory alloys of such metals, as well as, multiple layers of any ofsaid metals may also be used. The refractory-type metal is depositedpreferably sputtered over the base layer 12 to provide the conductivelayer 14'. The overcoat layer 18 is then chemically vapor deposited overthe conductive layer 14' without any significant degradation in thesurface morphology of the layer. Such a layer 14' may be patterned priorto depositing of the overcoat layer 18 to provide a conductive array forheating of the optical element 10, for example. Accordingly, when layer14' is fabricated to be a patterned metallic layer, suitable powersupply means (not shown) may be attached to said layer to provide theoptical element having an internally disposed heating arrangement. Suchelement may be used as an infrared window, for example, having ade-icing capability and, accordingly, would prevent defraction andreflection of incident infrared energy upon said window. Alternatively,the layer 14' may be continuous and used as a reflective layer

Examples of the optical element class described in conjunction with FIG.3 were fabricated having the single conductive/reflective layer 14'.Examples in which the conductive/reflective layer was "masked" had aconductive/reflective layer disposed only over central portions of thebase layer and with the overcoat layer disposed over peripheral portionsof the base layer and the conductive/reflective layer. All otherexamples have the conductive/reflective layer disposed over peripheraland central portions of the base layer.

    ______________________________________                                        Material of layers 17, 18                                                                             ZnSe                                                  CVD Deposition Temperature for layer 18                                                               700° C.                                        Thickness range of layers 12, 18                                                                      0.1 in. to 0.2 in.                                    ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                                                             % R after                                Example                                                                              Material of                                                                             Thickness   Initial % R                                                                           overcoat @                               No.    Layer 14  (Å)     @ 10μ                                                                              10μ                                   ______________________________________                                        2      Pt         800        83      62                                       3      Pt        1000        85      54                                                                            (curve 34)                               4      Pt        1000        85      54                                                        (masked)                                                     5      Mo         750        46      67                                       6      W         1000        N/A     72                                       7      Pt        1500        87      56.5                                     8      Pt        1500        87      53.5                                                      (masked)                                                     9      Mo        1000        43      59.5                                     10     Mo        1000        43      56.8                                     11     Mo        1500        41      56.2                                     12     Mo        1500        41      58                                       ______________________________________                                    

For a conductive layer comprising refractory types of metals such as Moand W, %R is typically 40-60% prior to overcoat with ZnSe. It isbelieved that the low initial % R was partially due to oxidation of thefilms during evaporation. Sputtered films or more carefully depositedevaporated films may have had a higher initial %R and thus a higher %Rafter CVD. As shown in Table 2, the %R was enhanced after CVD ofovercoat layer I8 for refractory metals such as MO and W. It is believedthat this increase was a result of grain growth, annealling and possiblechemical reduction of the refractory metal layer.

The percent reflectance (%R) is shown in FIG. 9A, curve 61 for the Ptlayer prior to the overcoat of zinc selenide and curve 62 subsequent tohaving overcoat layer 18 deposited thrreon. As can be seen, %R isreduced between 30-40% after overcoat of ZnSe.

                  TABLE 3                                                         ______________________________________                                                                             % R after                                Example                                                                              Material of                                                                             Thickness Initial % R @                                                                           overcoat @                               No.    layer 14' (Å)   10μ    10μ                                   ______________________________________                                        13     V         750       75        17.5                                     14     Pd        2000      88        23                                       15     Cr        800       73        18.5                                     16     Au/Cr     523 Å/60 Å                                                                      97        22.5                                     ______________________________________                                    

As shown in Table 3, less refractory types of metals such as Pd, V andCr, as well as, a Cr/Au alloy were unsuccessful at 700° C. temperature.However, for other CVD materials besides ZnSe or lower depositiontempertures for ZnSe, these materials may provide a higher post overcoatpercent reflectance (%R).

A second class of windows 11' were fabricated comprising zinc sulfide.

    ______________________________________                                        Material of layers 12, 18 ZnS                                                 Deposition Temperature for layer 18                                                                     °C.                                          Thickness range for layers 12, 18                                                                       0.125 in.                                           ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        Example Material of             % R after                                     No.     Layer 14    Thickness Å                                                                           overcoat @ 10μ                             ______________________________________                                        17      Mo (patterned)                                                                            1000        >85%                                          ______________________________________                                    

As is shown in Table 4, the % reflectance for a buried Mo layer is inexcess of 85%. Again, this %R is actually higher for Mo over thiswavelength range than the initial %R for Mo prior to the overcoat layerbeing deposited.

Referring now to FIG. 4, a second optical element 10, here an infraredwindow 11", is shown to include the substrate or base layer 12, acomposite intermediate layer 15, and the second or overcoat layer 18,here layers 12 and 18 each comprising a chemically vapor depositedmaterial. Here the composite layer 15 includes a conductive/reflectivelayer 15b disposed between a pair of passivating layers 15a, 15c, asshown. Passivation layers 15a and 15c are selected to substantiallyprevent diffusion or reaction between either the base layer 12 or theovercoat layer 18 and the conductive/reflective layer 15b. Further,passivation layer 15c is also provided to isolate theconductive/reflective layer 15b from the generally chemically reducingor oxidizing and corrosive environment provided during chemical vapordeposition of the overcoat layer 18. Accordingly, the thicknesses oflayers 15a and 15c are determined by the minimum thicknesses at whch thelayers are continuous and free of pinholes and other defects so that thesandwiched conductive layer 15b is substantially or preferablycompletely isolated from the base and overcoat layers 12, 18. Althoughthe thickness of the layer must have a minimum thickness of typically 50Å to act as passivation layers, generally, they should for manyapplications have a maximum thickness not exceeding 1000 Å to preventthe optical characteristic of the passivating layers 15a, 15c fromdominating over or superseding the optical characteristic of theconductive layer 15b. Passivation layers 15a and 15c may comprise one ofthe aforesaid mentioned refractory materials, such as one of theborides, carbides, nitrides, oxides or silicides. More particularly, thematerial may comprise a material such as BeO, AL₂ O₃, SiO₂, ThO₂, Y₂ O₃or ZrO₂, with BeO here being the preferred material. Further, certainrefractory-type metals having a relatively high melting temperature andhigh corrosion resistance, such as W, Mo, Ta, Ti and Rh may also beused, as well as, certain metal alloys having such properties. Theconductive/reflective layer may comprise a metal such as one of theaforesaid refractory metals and preferably a metal such as aluminum (AL)or one of the highly conductive/reflective metals, such as copper (Cu),silver (Ag), gold (Au), platinum (Pt) and palladium (Pd). Here platinumis the preferred metal for conductive/reflective layer 15b. Platinum hasa higher resistance to agglomeration than the other so-called highlyconductive/reflective metals and has a higher (%R), in particular, foroptical energy having a wavelength less than about 18 microns, than theso-called refractory type metals.

Examples of an optical element 11" having composite intermediate layer15 were fabricated. Each element included the base 12 of zinc selenide,a passivation layer 15a, a codncutive/reflective layer 15b, a secondpassivation layer 15c, and an overcoat layer 16 comprising a layer ofzinc selenide. The composite layer 15 is provided by sputtering thefirst passivating layer, the metallic reflective layer and the secondpassivating layer over the base layer. After the passivating layer 15cis provided, the overcoat layer 18 is chemically vapor deposited overthe passivation layer 15c.

                  TABLE 5                                                         ______________________________________                                        Layers 12, 18 ZnSe;                                                           Layer 18 deposited @ 700° C.                                           Thickness of Layers 12, 18 0.1 to 0.2 inches                                  material/thickness      % R reflectance                                       Example                                                                              Layer    Layer      Layer  after overcoat @                            No.    15a (Å)                                                                            15b (Å)                                                                              15c (Å)                                                                          10μ                                      ______________________________________                                        18     Mo 300   Pd 1000    Mo 300 68                                          19     Mo 300   Au 1000    Mo 300 56                                          20     BeO 1000 Pt 2000    BeO 1000                                                                             88                                          21     BeO 1000 Pt 1000    BeO 1000                                                                             94                                          22     BeO 500  Pt/W 1000 Å                                                                          BeO 500                                                                              92                                                          (50/50                                                                        Atomic %)                                                     23     W 150    Pt 1000    W 150  87                                          ______________________________________                                    

As previously described in conjunction with Table 2 and FIG. 9 curves61-62, if Pt alone is deposited and then overcoated or buried withinzinc selenide, there is a 30 to 40 percent decrease in the initialreflectivity of the Pt layer over the wavelength of range of 2-14microns. Accordingly, the single, unpassivated Pt layer provides arelatively poor reflective/conductive layer. However, as also shown inFIG. 9 curve 63, if the platinum film 15b is passivated on both sides bypassivating layers 15a, 15c, the reflectivity of the platinum istypically in eccess of 90 percent at wavelengths between 8-12 microns.That is, the reflectivity of the passivated platinum layer buriedbetween the pair of zinc selenide layers 12, 18 is actually higher thanthe reflectivity of the platinum layer prior to deposit of the secondpassivating layer as shown in FIG. 9 curve 61. It is believed that thisincrease in reflectivity of platinum layer 15 after the passivationlayer 15c and overcoat layer 18 are deposited, results from annealingand grain growth of the platinum layer 15b between the passivationlayers at the elevated temperatures occurring during chemical vapordeposition of the zinc selenide.

Referring now to FIG. 5, a plate-type dichroic beam splitter 31 is shownto include the base layer 12, an intermediate region 13 here comprisinga reflective layer 14', as described in conjunction with FIG. 3, or acomposite layer 15 as described in conjunction with FIG. 4, the overcoatlayer 18 here comprising a suitable chemically vapor deposited material,here of zinc selenide, and a dichroic reflective coating 20 depositedover a surface of the overcoat layer 18, as shown. An incident beam 40of optical electromagnetic energy impinges upon the dichroic reflectivecoating 20 at an incident angle α of nominally 45°. Here the incidentenergy represented by ray 40 includes high intensity radiation from aCO₂ laser (not shown), for example, and low intensity incoherent longwavelength infrared radiation (LWIR). The coating is selected to besubstantially totally reflective of the high intensity CO₂ laser energyand substantially totally transparent to the incoherent LWIR energy. Afirst ray 42 representative of the high intensity energy from the CO₂laser is reflected off the surface of the dichroic reflective coating20. The remaining portion of said incident radiation, here theincoherent LWIR radiation propagates through the dichroic coating andovercoat layer 16, and is reflected at the surface of the buriedreflective layers 14' or 15. The reflected energy will propagate backthrough the overcoat layer 18 and coating 20 emerging from the beamsplitter as ray 44. Thus, dichroic beam splitter 31 separates or splitsincident high intensity CO₂ laser energy from low intensity iccoherentLWIR energy. Other elements such as cubic beam splitters or intermediatelayers having a Fresnel structure, a prism or a reflective layer havinga surface not parallel with the surface of the dichroic coating may beused to provide additional angular separation of the incident energy.

Referring now to FIGS. 6, 7 and 7A, an infrared detection system 35(FIG. 6) is shown to include an infrared window 32 and an infraredutilization system 34. Incident infrared electromagnetic energy isprovided through the infrared window 32 to be utilized by the infraredutilization system 34 in a known manner. Further, RF energy 35 may beincident upon the infrared window 32 from a distant emttter (not shown).The incident RF energy 35 may be either reflected by the infrared windowas described in conjunction with FIGS. 3, 4 or will be absorbed by theinfrared window 32 as will now be described. The infrared window 32 isshown to include the base layer 12, here comprising a chemically vapordeposited material, here zinc selenide and a patterned composite layer15'. As shown in FIGS. 7, 7A, the composite layer 15' includes the firstpassivating layer 15a disposed over the substrate layer 12, a patterned,conductive layer 15b' disposed over the passivating layer 15a and asecond passivating layer 15c', here disposed over and between theunderlying patterned metallic layer 15b', as shown. The patternedmetallic layer 15b' may be used for heating the infrared window fordeicing purposes, as well as, providing absorption of incident radiofrequency energy 34 incident upon the window 31'. The conductive layer15b is deposited over the passivating layer 15a and is then masked andpatterned using conventional techniques to provide a plurality of herespaced conductors 15b". Here each of said conductors are spaced by adistance substantially greater than several hundred or thousandwavelengths of the corresponding incident optical energy. With such anarrangement, the conductors will be spaced relatively closely forincident r.f. energy and such energy will be absorbed, whereas, theconductors will be spaced relatively far apart compared to thewavelength of the incident optical energy and accordingly the conductivelayer 15b' will be transparent to such optical energy.

Having described preferred embodiments of the present invention, it willnow be apparent that other embodiments incorporating its concept may beused. It is felt, therefore, that this invention should not be limitedto the disclosed embodiment, but rather should be limited only by thespirit and scope of the appended claims.

What is claimed is:
 1. An article comprising:a base comprising anoptically transmissive material; an intermediate region disposed over atleast a portion of the base comprising a layer of a refractory type ofmaterial; an overcoat having a physical thickness greater than about 25microns disposed over the intermediate layer comprising a denseoptically transmissive material having an actual density substantiallyequal to 100% of the theoretical density of said material.
 2. Thearticle of claim 1 wherein the refractory material comprises at leastone material selected from the group consisting of molybdenum, tungsten,tantalum, titanium, rhodium, a refractory metal alloy, one of theborides, carbides, nirrides, oxides or silicides.
 3. The article ofclaim 2 wherein the refractory material comprising at least one materialselected from the group consisting of molybdenum, tungsten, tantalum,titanium and rhodium.
 4. The article of claim 3 wherein said basmaterial has an actual density substantially equal to 100% of thetheoretical density of said material.
 5. The article of claim 2 whereinthe refractory material comprises at least one material selected fromthe group consisting of the borides, carbides, nitrides, oxides andsilicides.
 6. The article of claim 5 wherein the refractory materialcomprises at least one material selected from the group consisting ofBeO, Al₂ O₃, SiO₂, ThO₂, Y₂ O₃ and ZrO₂.
 7. The article of claim 5wherein said base material has an actual density substantially equal to100% of the theoretical density of said material.
 8. The article ofclaim 1 wherein the intermediate region further comprises a layer of aconductive material disposed on the layer of refractory material and asecond layer of a refractory type of material disposed to cover theconductive layer, and wherein the overcoat layer is disposed over thesecond layer of the refractory material.
 9. The article of claim 8wherein the physical thickness of the overcoat is greater than about 25microns.
 10. The article of claim 9 wherein the conductive layercomprises at least one material selected from the group consisting oftungsten, molybdenum, aluminum, copper, gold, silver, platinum andpalladium.
 11. The article of claim 10 wherein the conductive layercomprises at least one material selected from the group consisting ofpalladium, platinum and gold.
 12. The article of claim 9 wherein eachrefractory material comprises at least one material selected from thegroup consisting of molybdenum, tungsten, tantalum, titanium, arefractory metal alloy, the borides, carbides, nitrides, oxides andsilicides.
 13. The article of claim 12 wherein each refractory materialcomprises at least one material selected from the group consisting ofmolybdenum, tungsten, tantalum, titanium, and rhodium.
 14. The articleof claim 12 wherein each refractory material comprises at least onematerial selected from the group consisting of the borides, carbides,nitrides, oxides and silicides.
 15. The article of claim 14 wherein eachrefractory material comprises at least one materia selected from thegroup consisting of BeO, Al₂ O₃, SiO₂, ThO₂, Y₂ O₃ and ZrO₂.
 16. Thearticle of claim 8 wherein each refractory material comprises at leastone material selected from the group consisting of molybdenum, tungsten,tantalum, titanium, rhodium, a refractory metal alloy, the borides,carbides, nitrides, oxides and silicides.
 17. The article of claim 16wherein each refractory material comprises at least one materialselected from the group consisting of molybdenum, tungsten tantalum,titanium, and rhodium.
 18. The article of claim 11 wherein eachrefractory type of material comprises at least one material selectedfrom the group consisting of the borides, carbides, nitrides, oxides andsilicides.
 19. The article of claim 18 wherein said base material has anactual density substantially equal to 100% of the theoretical density ofsaid material.